Effect of pH on the Photophysical and Redox Properties of a Ruthenium(II) Mixed Chelate Derived from Imidazole-4,5-dicarboxylic Acid and 2,2′-Bipyridine: An Experimental and Theoretical Investigation

Combined experimental and DFT–TD-DFT computational studies were utilized to investigate the structural and electronic properties of mixed-ligand monometallic ruthenium(II) complexes of compositions [(bpy)2Ru(H2Imdc)]+ (1+), its N–H deprotonated form [(bpy)2Ru(HImdc)] (1), and COOH deprotonated form [(bpy)2Ru(Imdc)]− (1–), where H3Imdc = imidazole-4,5-dicarboxylic acid and bpy = 2,2′-bipyridine. The optimized geometrical parameters for the complexes computed both in the gas phase and in solution are reported and compared with the previously reported X-ray data. The influence of pH on the absorption, emission, and redox properties of [(bpy)2Ru(H2Imdc)]+ (1+) has been thoroughly investigated. The absorption titration data were used to determine the ground state pK values, whereas the luminescence data were utilized for the determination of excited state acid dissociation constants. The proton-coupled redox activity of 1+ has been studied over the pH range 2–12 in acetonitrile–water (3:2). From the E1/2 versus pH profile, the equilibrium constants of the variously deprotonated complex species in RuII and RuIII oxidation states have been determined. As compared to the protonated complex (1+), which undergoes reversible oxidation at 0.96 V (vs Ag/AgCl) in acetonitrile, the redox potential of the fully deprotonated complex (1–) is shifted to a much lower value, viz., 0.52 V. Density functional theory (DFT) and time-dependent DFT (TD-DFT) study provides insight into the nature of the ground and excited states with resulting detailed assignments of the orbitals involved in absorption and emission transitions. In particular, the red-shifts of the absorption and emission bands and the cathodic shift in the oxidation potential of 1+ compared to 1 and 1– are also reproduced by our calculations.